Re: [boost] [system] Was: Re: [asio] Breaking changescommittedtocvs HEAD

From: Christopher Kohlhoff (chris_at_[hidden])
Date: 2006-11-16 07:37:11

Hi Johan,

Johan Nilsson <r.johan.nilsson_at_[hidden]> wrote:
> I'd be happy enough if such wrappers would be documented and
> existed in <boost/system/convenience.hpp> or similar.

Here's the sort of convenience wrapper that I would be happy to
see (just a 4-argument Windows version that I hacked up):

template <
    typename R,
    typename A1,
    typename A2,
    typename A3,
    typename A4>
R system_call(R (__stdcall *f)(A1, A2, A3, A4),
    typename boost::mpl::identity<A1>::type a1,
    typename boost::mpl::identity<A2>::type a2,
    typename boost::mpl::identity<A3>::type a3,
    typename boost::mpl::identity<A4>::type a4,
    boost::system::error_code& ec,
    typename boost::mpl::identity<R>::type failure_retval = R())
{
  R r = f(a1, a2, a3, a4);
  if (r == failure_retval)
  {
    DWORD last_error = ::GetLastError();
    ec = boost::system::error_code(
        last_error, boost::system::native_ecat);
  }
  else
  {
    ec = boost::system::error_code();
  }
  return r;
}

template <
    typename R,
    typename A1,
    typename A2,
    typename A3,
    typename A4>
R system_call(R (__stdcall *f)(A1, A2, A3, A4),
    typename boost::mpl::identity<A1>::type a1,
    typename boost::mpl::identity<A2>::type a2,
    typename boost::mpl::identity<A3>::type a3,
    typename boost::mpl::identity<A4>::type a4,
    typename boost::mpl::identity<R>::type failure_retval = R())
{
  boost::system::error_code ec;
  R r = system_call(f, a1, a2, a3, a4, ec, failure_retval);
  if (ec)
    throw boost::system::system_error(ec);
  return r;
}

And you can use it like this:

  system_call(::MessageBox, hwnd, "A", "B", MB_OK);

or:

  boost::system::error_code ec;
  system_call(::MessageBox, hwnd, "A", "B", MB_OK, ec);

It seems pretty convenient to use, does not have the issues with
thread-specific error codes that I am concerned about, and
provides a portable interface over the top of platform-specific
error handling APIs. It'll even save people from making the
error of forgetting to set the error_code out parameter 'ec'.

Cheers,
Chris